The disclosure of Japanese Patent Application No. 2000-399862 filed on Dec. 28, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a speed shift control apparatus of an automatic transmission and, more particularly, to an art for smoothly performing a speed shift that needs release of two engageable elements and engagement of two different engageable elements (simultaneous engagement switching of two elements) for a shift from one gear speed to another.
2. Description of Related Art
An automatic transmission usually achieves a plurality of gear speeds through changes in gear ratio accomplished by changing a power transfer path extending via speed shift elements formed by planetary gears through engagement and release of friction engageable elements. In order to perform the engagement and release of engageable elements at the time of a speed shift by a simple hydraulic control while suppressing shift shock, the manipulation of engageable elements for upshifts or downshifts is generally performed by a basic operation in which a plurality of engageable elements or a single engageable element is set in the engaged state for a specific gear speed, while another engageable element is engaged or an engageable element in the engaged state is released. However, if the basic operation is not possible due to the gear train construction, an engagement-switching operation is performed in which an engageable element is released from an engaged state while another engageable element is engaged.
In recent years, there has been a trend toward an increase in the number of speeds of an automatic transmission due to demands for improved drivability and energy conservation. Generally, the number of gear speeds of an automatic transmission is increased through addition of a higher gear speed or a lower gear speed by adding an overdrive or underdrive gear to a speed shift mechanism formed by a plurality of planetary gear sets. Another technique of increasing the number of speeds is disclosed in Japanese Patent Application Laid-Open No. HEI 4-219553 in which multiple gear speeds are achieved by providing two systems, that is, high and low systems, of input to a Ravigneaux type planetary gear set.
In a gear train designed for multiple gear speeds, engagement-switching is a complicated operation of four elements instead of the simple engagement-switching operation of two elements due to the increased choices of gear speeds suitable for the state of running of the vehicle. An example in which the four-element engagement-switching operation is needed is a jump shift in which the gear speed is immediately shifted from one of many gear speeds to a specific gear speed. In any case, where an engagement-switching operation of four elements is performed, the control of the sequence and timing of engagement and release of engageable elements is very important. Depending on the fashion of such control, a smooth shift process that occurs in the speed shift mechanism may be impeded, and continuity of speed shift may be lost. In that case, various problems arise, such as, for example, stepwise shocks during a speed shift, a considerably increased shock at the end of a speed shift, a shift duration prolonged more than necessary, etc.
Accordingly, it is an object of the invention to provide a speed shift control apparatus of an automatic transmission capable of preventing a prolonged speed shift while eliminating occurrence of a shock during a speed shift by achieving a smooth progress of a speed shift if the speed shift requires the engagement and release of four engageable elements.